Method of making alloys of second rare earth series metals



ttes

Uite

tent

Patented Jan. 8, 1963 3,072,475 METHOD OF MAKING ALLOYS OF SECOND RARE EARTH SERIES METALS Richard D. Baker, Los Alamos, N. Mex., and Benjamin R. Hayward, Whittier, Califl, assignors to the United States of America as represented by the United States Atomic Energy Commission N Drawing. Filed Mar. 7, 1951, Ser. No. 214,442 7 Claims. (Cl. 75-1225) This invention relates to a method of making alloys of the second rare earth series metals; more particularly it relates to a method for making alloys of the second rare earth series with molybdenum, niobium or zirconium.

In the field of reactor design the necessity for high strength materials having the required nuclear properties is well known. Substantially pure uranium is often the optimum metal for use in neutron reactors and neutron multiplying systems, but the metal itself is diflicult to work and to fabricate because of its low mechanical strength. Alloys containing a large proportion by weight of uranium and a minor proportion by weight of other elements may be used in neutron reactors as long as the modifying element does not cause a material increase in the neutron capture cross section for the uranium alloy. Of particular interest in this field are the alloys of uranium with nobiurn, molybdenum and zirconium.

Alloys of uranium containing from 1 to atomic percent of zirconium, niobium or molybdenum possess desirable nuclear and other properties. The capture cross sections of these metals for neutrons at thermal energies are below that of natural uranium, i.e., about 3 barns, or 3 X10 square centimeters; uranium alloys containing the above percentages of these elements also have low capture cross sections. Any of these alloys may thus be conveniently used in a nuclear reactor without requiring an appreciably greater amount of the alloy than of pure uranium. In addition to the above properties these alloys possess increased strength over unalloyed uranium. However, for satisfactory results, it is highly important that they be homogeneous and possess a high degree of purity, and further, there must be available a suitable process for their production.

In the past, attempts have been made to produce these alloys by direct fusion with the aid of external heat and by the aluminothermic process. However, neither of these methods has proved entirely satisfactory as the alloys when produced on a large scale by these and other prior methods are porous and the constituents are segregated. Further, the high temperatures required make these methods impractical. Zirconium melts at 1900 C., and the melting points of niobium and molybdenum are 2500 C. and 2620 C., respectively. Efforts to place these metals in solution in uranium using conventional vacuum melting techniques have met With only limited success when production is attempted on a large scale. It appears that a higher temperature is necessary than can be easily obtained by present direct melting techniques.

Most of the prior processes which have been attempted for the production of the above alloys require that the metals themselves be used, thus necessitating the production of the metal from the oxide or other compound before the alloying process. In attempts to make alloys using the oxides of the metals so much difiiculty has been encounterd because of the presence of oxygen that the use of the oxide has not appeared feasible. Attempts have been made to produce an alloy of uranium and molybdenum from the oxides by the alumino-thermic process, but the product produced by this method is badly segregated and highly porous. A non-homogeneous alloy poses problems relating to fabricating, heat-treating, and working in general, in addition to the obvious and serious defect of non-uniformity.

It is therefore an object of this invention to provide a method for the production of homogeneous, high purity alloys having desirable nuclear and structural properties, namely, alloys of metals of the second rare earth series with molybdenum, niobium or zirconium.

It is another object of this invention to provide a method for producing the above alloys using the oxideof the metal which is to be alloyed with the second rare earth series metal.

It is a further object of this invention to provide a method for producing the above alloys which provides a high percentage yield.

It has been found that the above and other objects are accomplished by thoroughly mixing a halide of a second rare earth series metal with an oxide of Zirconium, niobium or molybdenum, a reducing agent consisting of an alkali metal or an alkaline earth metal, such as calcium, and a booster such as iodine, and heating the mixture in an inert atmosphere until reaction is initiated. In the reaction, the oxide and halide are co-reduced by the reducing agent to produce the metals, which are liquefied and alloyed by the high local internal heat resulting from the exothermal character of the reaction. Although it has proved impossible to measure the maximum temperature accurately because of its short duration, a temperature of 1700 C. has been noted and it appears that temperatures in excess of 2620 C., the melting point of molybdenum, are obtained at least locally. Heat to initiate the reducing reaction is furnished in part by the reaction between calcium and iodine, the preferred combination of reducing agent and booster, to form calcium iodide. By virtue of its low melting point, this material increases the fluidity of the slag and thus aids in the collection of the metal formed into a unitary mass, in which condition the alloy may be recovered from the cooled reaction mixture.

It is a feature of the process of this invention that the alloys produced thereby are in the form of buttons or unitary masses, and may therefore be recovered readily, simply and completely from the reaction mixture without the necessity for leaching processes or other troublesome or possibly deleterious treatments required for the recovery of finely-divided products, with consequent danger of damage to the alloy or of incomplete recovery thereof. It is to be noted, however, that thorough mixing of the charge has been found important to the production of such a button. If the charge is not completely blended, the button will be poorly formed and the alloy may be inhomogeneous.

The success of the present process is not in accord with the general practice in the uranium reduction art, where it has been found important to provide a uranium halide completely free from oxygen in order to obtain high yields in a reduction process such as the present process. While no complete understanding of the mechanisms here involved has yet been achieved, it has been found that satisfactory results are obtained by the process of this invention in preparing alloys containing up to the order of about 20 atomic percent of the alloying element. Thus, the process of this invention covers the range of from 1 to about 20 atomic percent of the alloying element. The upper limit is not a fixed one, and will vary according to variations in standards, procedures, techniques and materials. For example, operation on the relatively small scale of a charge of only a few grams will not permit the attainment of acceptable alloys of such high percentage of alloying element as will operation on a larger scale. The proportions of reducing agent and booster used may be varied to modify the behavior of the reaction mixture and the nature of the final product. Many other variations will be obvious to those skilled in the art, and the detailed nature and limits of the process can be affected thereby.

The following information regarding preparation and properties of uranium alloys is given to illustrate typical preferred embodiments of this invention. It is to be understood that halides of other elements of the second rare earth series such as thorium, plutonium and neptunium may be alloyed in a comparable manner, and that no limitation to uranium or to a fluoride is intended. The other halides, that is, chloride, bromide and iodide, may be satisfactorily employed and the metals may be in either the trivalent or tetravalent state.

A well-blended charge of the following finely divided components is used as the base charge in a series of experiments, selected amounts of the oxide of the alloying element being blended therewith as desired.

BASE CHARGE Grams Uranium tetra-fluoride 1319.2 Calcium metal 439.4 Iodine 106.6

After being thoroughly mixed, the complete charge, consisting of the base charge together with the desired amount of oxide of zirconium, molybdenum or niobium, is placed in a steel bomb with a refractory liner, typically vitrified magnesium oxide. The bomb is sealed, provided with an inert atmosphere such as argon, and heated in an induction furnace to an external temperature of about 550 C. This external temperature corresponds to an internal temperature, as shown by a previously calibrated thermocouple, of between 350 C. and 400 C., which is sufficient to initiate reaction between the calcium and the iodine. The booster reaction sets off the main reduction reaction, and a peak temperature is reached in about minutes after the beginning of the heating cycle.

After the bomb has cooled to room temperature, the contents are removed, and the slag and the refractory liner are broken away from the metal button. The button is cleaned by immersion in dilute acetic acid, and is then weighed and sectioned for examination.

Typical amounts of molybdenum oxide, zirconium oxide or niobium oxide added to a base charge are:

ADD-ED OX[DE Results of a program of tests on the above alloys and other similar alloys are given below. Tables I and III show data regarding the homogeneity of alloys prepared by the process of this invention. The yields of metal recovered based on materials charged were uniformly greater than 99 percent. The data on alloying element content, both as charged and as found by analysis of various portions of the button, indicated in the tables as A, B, or C, show clearly the homogeneous nature of the product. Data on so-me direct-fusion alloys are in cluded for comparison. In Tables II and. IV are given data regarding improvements in grain size of the alloys, and in hardness as shown by both Rockwell and diamond pyramid hardness tests.

Table I URANIUM-NIOBIUM ALLOYS [a. Corcduced alloys] Niobium Content Sample No. Charged Found (Wt. Percent) At. Wt. Per- Per- A B 0 cent cent Average 2.07 2.03 2.01

[b. Direct fusion alloys] Niobium Content (Wt. Percent) Sample No.

Charged Found Table II URANIUM-NIOBIUM ALLOYS Rockwell Grain Size Nb Content (Atomic Percent) Hardness (Mean Diam.

Microns) 80 RB 500 R n 150 100 R13 150 100 BB 45 R o 90 50 Table III URANIUM-MOLYBDEN UM ALLOYS Molybdenum Content Sample No. Charged Found (Wt. Percent) At. Wt. Percent Percent A B Average 2. 21 2. 33

Table IV URANIUM-MOLYBDENUM ALLOYS Diamond Grain Size Mo Content (Atomic Percent) Rockwell Pyramid (Mean Hardness Hardness Diam.

Microns) 80 RB 500 29 Re 293 500 33 Re 325 300 90 BB 150 27 Re 278 50 The above alloys are heat-treatable: such properties as tensile strength, corrosion resistance and resistance to compression are controllable by heat treatment. The alloys show an increased tensile strength over unalloyed uranium. The alloy containing 5 atomic percent of molybdenum after heat treatment has an ultimate strength in excess of 200,000 pounds per square inch as compared to 65,000 pounds per square inch for uranium. Comparable strength features are possessed by the uranium-niobium alloys produced by the above process. As previously indicated, this strength factor is highly important in reactor development.

Changes in composition of the alloys are accompanied by corresponding changes in properties. The composition of the alloy can be made highly flexible below the aforementioned upper limit of the order of about 20 atomic percent, as it is possible to produce with the above process either an exact alloy of a predetermined composition or a master alloy which can be diluted to the required percentage. One of the major ditiiculties connected with the direct fusion process is the fact, as indicated by the results in Table 112, that it is diflicult to obtain an alloy approaching the desired alloy composition. In contrast, the results in Tables Ia and 111 show that by the present process an alloy of the predicted composition is obtained within analytical limits. The importance, especially in nuclear work, of having available a process which wili produce an alloy of a predicted composition is obvious.

Photomicrographic examination of the above samples shows conclusively that a true alloy is formed. This study verifies the presence of homogeneous alloys rather than inhomogeneous mixtures. Analyses of these alloys reveal a purity consistently greater than 99.9 percent. Unlike analyses of alloys formed by direct fusion, the analyses of the above alloys show no indication of insolubility in any case: in contrast to direct fusion alloys, they are completely soluble in hydrochloric acid.

While preferred embodiments of the invention have been described, it will be apparent to those skilled in the art that this invention is not necessarily limited to the particular embodiments described herein, but that various modifications may be made without departing from the scope of the invention as set forth in the appended claims.

We claim:

1. The process of making alloys comprising at least one metal from the class consisting of metals of the second rare earth series and a total of from 1 to about 20 atomic percent of at least one metal from the class consisting of zirconium, niobium and molybdenum which comprises thoroughly mixing an oxide of at least one metal from the second class with a halide of at least one metal from the first class, iodine and a reducing agent from the class consisting of alkali metals and alkaline earth metals and heating the mixture in an inert atmosphere to a temperature of about 350 C. whereby a reduction reaction is initiated.

2. The process of making alloys comprising one metal from the class consisting of uranium and transuranic elements and from 1 to about 20 atomic percent of one metal from the class consisting of zirconium, niobium and molybdenum which comprises thoroughly mixing a halide of a metal from the first class with a charge comprising an oxide of a metal or" the second class, iodine, and a reducing agent chosen from the class consisting of alkali and alkaline earth metals and heating said mixture in an inert atmosphere to a temperature of about 350 C. whereby a reduction reaction is initiated.

3. The process of claim 1 in which the halide is uranium tetrafluoride, the oxide is niobium oxide and the reducing agent is calcium.

4. The process of claim 1 in which the halide is uranium trichloride, the oxide is molybdenum oxide and the reducing agent is calcium.

5. The process of claim 1 in which the halide is uranium tribromide, the oxide is zirconium oxide and the reducing agent is calcium.

6. The process of claim 1 in which the halide is plutonium tetrafiuoride, the oxide is niobium oxide and the reducing agent is calcium.

7. The process of claim 1 in which the halide is neptunium trichloride, the oxide is molybdenum oxide and the reducing agent is calcium.

References Cited in the file of this patent UNTTED STATES PATENTS 875,345 Goldschmidt Dec. 31, 1907 1,019,394 Weintraub Mar. 5, 1912 1,306,568 Weintraub June 10, 1919 1,648,954 Marden Nov. 15, 1927 1,728,940 Marden Sept. 24, 1929 1,728,941 Marden Sept. 24, 1929 1,814,721 Marden July 14, 1931 OTHER REFERENCES Lilliendahl: The Electrochemical Society, preprint 91- 16, pages 237-246 (1947). (Copy in Div. 3.)

Mellor: Modern Inorganic Chemistry, 1939 edition, page 527, published by Longmans, Green and Company, London. (Copy in Scientific Library.) 

1. THE PROCESS OF MAKING ALLOYS COMPRISING AT LEAST ONE METAL FROM THE CLASS CONSISTING OF METALS OF THE SECOND RARE EARTH SERIES AND A TOTAL OF FROM 1 TO ABOUT 20 ATOMIC PERCENT OF AT LEAST ONE METAL FROM THE CLASS CONSISTING OF ZIROCONIUM, NIOBIUM AND MOLYBDENUM WHICH COMPRISES THOROUGHLY MIXING AN OXIDE OF AT LEAST ONE METAL FROM THE SECOND CLASS WITH A HALIDE OF AT LEAST ONE METAL FROM THE FIRST CLASS, IODINE AND A REDUCING AGENT FROM THE CLASS CONSISTING OF ALKALI METALS AND ALKALINE EARTH METALS AND HEATING THE MIXTURE IN AN INERT ATMOSPHERE TO A TEMPERATURE OF ABOUT 350*C. WHEREBY A REDUCTION REACTION IS INITIATED. 